Throw errors when blockNumber is invalid in ChainRpc

Currently, `getBalance` and `getNonce` are able to resolve `null` when no such block exists. (the given blockNumber is too big)
For TypeScript users, they should put the type guard for every `getBalance` and `getNonce`.

The problem is, a type guard is useless if the users use a valid block number only.

Methods using blockNumber parameter
- getBalance
- getNonce
- getRegularKey
- getRegularKeyOwner
- getAsset
- getAssetScheme*
- getBlock
- getBlockHash
- isAssetSpent